Photographic elements employ emulsions consisting of small crystals of silver halide grains suspended in a colloidal polymeric matrix, typically gelatin. Silver halide crystals are semi-conductors, and inherently absorb light in the blue region of the visible spectrum. Sensitivity to all regions of the light spectrum (which term is used here to include infra-red) may be imparted to silver halide crystals by adsorbing spectral sensitizing dyes to the silver halide crystals. Spectral sensitizing dyes are designed to absorb light in specific regions of the spectrum. This feature is useful in photographic elements which have one or more emulsion layers sensitive to the same region of the spectrum (for example, black and white or X-ray elements). More particularly though, it allows color films with three channels to be constructed, where each channel is sensitive to one of the three regions of visible light--blue, green, or red.
Desirable properties of sensitizing dyes include a strong affinity to adsorb to a silver halide surface, and the ability to sensitize efficiently the silver halide grain to specific portions of the spectrum. It is also desirable that the dyes be removed from film during processing to prevent undesirable dye stain in the processed film. Most typical spectral sensitizing dyes are members of the cyanine and related dye classes, often bearing solubilizing groups such as sulfo or carboxylate groups. Sensitizing dyes which absorb red light are typically carbocyanines or merodicarbocyanines; complex cyanines and complex merocyanines may also be used. The solubilizing groups are present to aid dissolution of the dye for coating on silver halide, and removal during processing. Many of the typical red spectral sensitizing dyes used in silver halide systems leave dye stain in part because they are or become colored, and are not fully removed during processing.
In addition to dye stain, another undesirable photographic feature which accompanies the use of spectral sensitizing dyes is a phenomenon called desensitization. Desensitization is described in the text The Theory of the Photographic Process, T. H. James, editor, 4th Edition, Macmillan, New York, 1977. Generally, the adsorption of a sensitizing dye to a silver halide grain may reduce the efficiency of the intrinsic response of the grain to blue light; a concomitant loss of efficiency of the spectrally sensitized response also occurs. The effect increases as the amount of adsorbed dye increases. Dyes vary in the extent of desensitization caused at a given dye load, but all dyes will desensitize when adsorbed at sufficiently high levels. Desensitization is undesirable in conventional photographic systems. By reducing the efficiency with which photons are converted to developable latent image, the effectiveness with which the sensitized emulsion may be employed in a photographic element is also reduced.
In contrast to sensitizing dyes described above, filter dyes are used in photographic films and paper to absorb light which would adversely impact the coated silver halide emulsions. Applications of photographic filter dyes include interlayer light filtration, i.e., the traditional yellow filter dyes and magenta trimmer dyes, and intralayer light filtration. Filter dyes generally must be coated at much higher levels than sensitizing dyes to perform their desired function. Functionalization of dyes with solubilizing groups is frequently not sufficient to fully remove them from film during processing. For this reason, filter dyes which must be removed from the film upon processing are designed to be bleachable or decolorizable under processing conditions. One class of useful filter dyes is that described in U.S. Pat. No. 5,213,956. However, the dyes of that patent were solely described for use as filter dyes.
The structural features of the dyes designed for spectral sensitization often exclude them from performing practically as filter dyes and vice versa. Traditionally, different dye classes and structures have been employed in photographic systems to achieve the separate tasks of spectral sensitization and light filtration.
Hence, it would be highly advantageous to design sensitizing dyes which effectively sensitize silver halide emulsions as traditional sensitizing dyes do, yet decolorize upon processing as many traditional filter dye structures do. Additionally, it would be a useful if sensitizing dyes could be found which desensitized to a lesser extent when coated at equal levels than dyes which are customarily used in the practice of spectral sensitization of silver halide emulsions.